A supressor mutation in Escherichia coli

Nutritionally deficient mutant strains of bacteria (auxotrophs) are often capable of undergoing "reversion," giving rise to strains having the wild type nutritional state (prototrophs). It is usually impossible to determine whether the prototrophs are true back-mutants, or whether the wild phenotype is restored by the action of an independent suppressor mutation. An auxotroph requiring histidine was isolated from strain B/r of Escherichia coli (which grows well on minimal medium) after ultraviolet irradiation and layering(1). Some time later, the histidineless strain was irradiated with ultraviolet and subjected to penicillin screening(2,3) Among the mutants isolated was one requiring serine or glycine in addition to histidine. This diauxotroph, strain M2, was thus characterized by two growth factor requirements acquired separately and serially, presumably by two mutational steps